The field of the present invention relates generally to a plasma enhanced chemical vapor deposition (CVD) reactor. In particular, the field of the present invention relates to a plasma CVD device suitable for the synthesis of materials such as diamond, boron nitride, boron carbide, ceramics containing oxides, nitrides, carbides and borides, or the like, and for the deposition of a uniform thin film layer of gallium nitride, or the like, over a substrate.
Diamond and other high temperature materials such as boron nitride, boron carbide, and ceramics containing oxides, nitrides, carbides and borides are finding increasing uses because of their high temperature and wear resistance, and high strength. In particular, there is increasing demand for coatings of these materials to be deposited on ordinary materials to impart wear resistance, abrasion resistance, and high temperature resistance.
A conventional plasma enhanced CVD reactor employs microwaves to produce a stable plasma. The apparatus consists of a microwave generator, a tuning element, a waveguide, and a quartz tube. The quartz tube is passed through the waveguide, and a substrate is placed inside the quartz tube in the region where it passes through the waveguide. Suitable gases are introduced into the tube, for example in the case of diamond synthesis, a mixture of methane and hydrogen. The plasma is formed inside the tube by the microwave radiation, and a diamond layer is deposited on the substrate surface. The substrate size is limited to very small sizes by the physical dimensions of the microwave cavity.
A second conventional technique also employs microwaves to produce stable plasmas. The apparatus consists of a microwave cavity, a vacuum chamber connected to the microwave cavity by means of a microwave transparent window, and a microwave generator and waveguide to introduce microwaves into the microwave cavity. A magnetic field is generated by magnets external to the microwave cavity. The magnets have a polarity and magnitude suitable to create an electron cyclotron resonance (ECR) condition within the chamber. This technique has a disadvantage in that the pressure of the gas is limited to 0.5 torr or less by the necessity of maintaining the electron cyclotron resonance condition. Accordingly, the deposition rates of diamond materials are very low.
A third type of conventional plasma reactor also employs microwaves to produce stable plasmas. The type of plasma reactor is represented by U.S. Pat. No. 4,940,015. The apparatus consists of a microwave cavity, a vacuum chamber connected to the microwave cavity by means of a microwave transparent window, and a microwave generator and waveguide to introduce microwaves into the microwave cavity. The chamber is of suitable dimensions so as to create a cavity resonance condition for the efficient absorption of microwave energy by the plasma. However, it is not possible to move the plasma relative to the substrate surface, and hence the substrate size is limited.
Conventional methods of coupling an E field into a microwave cavity such as is shown in U.S. Pat. No. 4,866,346 employ a mode coupler. See for example, the '346 patent at col. 7, lines 42-43. A conductive rod passes from an input waveguide into a cylindrical cavity, characterized as an output waveguide. The conductive rod passing between the waveguides is a mode coupler. This configuration has the disadvantage of limiting substrate size. Also, the mode coupler exhibits inferior plasma stability. Consequently, the plasma cannot be closely controlled to form a uniform coating of a diamond or refractory material.
As will be explained, the present invention uses an antenna radiator, rather than a microwave coupler, for radiating an E field of maximized intensity into a stable microwave cavity. This provides an E field of maximized intensity which produces a plasma that can be closely controlled to precise tolerances.
It is not possible with conventional plasma enhanced CVD reactors to control the plasma to the degree necessary for certain new semiconductor processing applications such as, for example, deposition of diamond films. It would be advantageous to grow single crystal thin film diamonds in a CVD process for electronic applications. The presently known conventional plasma enhanced CVD techniques described above are unable to produce single crystal diamond films which are large enough for electronic devices.
Single-crystal thin film diamonds promise broad utility in electronic applications. Diamond materials for electronics consist of a single crystal structure and contain doping materials to make them semiconductive.
A diamond material is unique in that it has higher thermal conductivity than any other material at room temperature and above. Thus, diamond circuits could be more stable than conventional semiconductor circuits and remove accumulated heat faster. Another advantageous property of diamond is its large energy band gap, 5.45 electron volts, as compared to only 1.1 electron volts for silicon.
The large band gap of a diamond would enable it to operate at extremely high voltages and at high currents. Diamond electronic devices also can operate at high frequencies. A diamond device could run at speeds of 300 GHz as opposed to 10 MHz for the device that currently powers the IBM PC-AT.
The foregoing properties enable electronic devices made with diamond to work faster, withstand more power and fit closer together than current devices. This would also mean that electronic circuits based upon diamond materials could form the basis for extremely high-speed computing devices. Because diamond is harder and more durable than any other semiconducting material, diamond circuits will resist harsh environments that would melt or corrode existing semiconductors.
The use of diamonds in electronics is currently limited by the inability of conventional plasma enhanced CVD devices to produce uniform, thin film diamond coatings in sufficient quantity over a large enough area to make a diamond based electronic device commercially practical.
Conventional plasma enhanced CVD devices have limitations in producing diamond coatings which arise from their inability to precisely control the plasma with respect to the substrate being coated. The nature of the plasma depends upon many independent variables such as electron concentration, electron-energy distribution, gas density and so forth. It has not been possible with conventional plasma enhanced CVD devices to control these variables to a sufficient degree to produce a single crystal diamond coating over a large wafer in commercially feasible quantities. Conventional devices also lack the ability to control the plasma to the extent necessary to produce single-crystal diamond films with sufficient uniformity to be used in commercial quantities.
Another major drawback of current CVD deposition techniques is the requirement for abrasive treatment of substrate surfaces prior to deposition to promote nucleation and growth of continuous polycrystalline diamond layers. Although bias enhanced nucleation has been attempted, it is not apparent that this technique can be applied to non-conducting substrates.
Therefore, what is needed is an improved plasma enhanced CVD apparatus which is capable of optimizing the plasma to substrate contact to the degree necessary to provide diamond synthesis in commercial quantities.
What is also needed is an improved plasma enhanced CVD reactor capable of producing single crystal thin film diamond with requisite uniformity and in wafers large enough to be commercially practicable for electronic devices.
It would also be advantageous to provide an improved plasma enhanced CVD reactor capable of rapid thermal processing for producing large quantities of uniform thin film coatings of materials such as gallium nitride or the like over substrates at a greater rate than was previously possible.
What is also needed is a method for forming a diamond coating or the like which contains as an integral step within the same system, a process for forming a nucleation layer on which the diamond will grow by itself. What is also needed is a process for forming a nucleation layer, integral with a system for forming a diamond coating, wherein the process for forming a nucleation layer does not rely on the use of substrate bias and is applicable to non-conducting substrates, as well as to conducting substrates.
It would also be advantageous to incorporate the process for forming a nucleation layer integrally with the same system for creating a diamond layer in order to further enhance diamond growth and to eliminate the need for providing two separate systems, one for creating a nucleation layer, as a base for diamond growth, and another system for depositing the diamond coating.